Dimethyl carbonate (hereinafter, also referred to as DMC) is a very useful compound as a fuel additive for increasing the octane value of gasoline etc., as a raw material for polycarbonate, which is an engineering plastic, as a carbonylating agent, or the like. Further, the use of dimethyl carbonate as a raw material for a fuel cell is anticipated. Therefore, the synthesis of dimethyl carbonate has been attempted by using various methods.
As a method for synthesizing dimethyl carbonate, there has conventionally been known a method for obtaining dimethyl carbonate by means of a reaction under a high pressure condition using carbon dioxide and methanol as raw materials. However, because of water yielded at the time of reaction, this method has a problem of low yields and selectivity.
Also, as an industrial manufacturing method, there has generally been known a process for obtaining dimethyl carbonate by means of an esterification reaction using phosgene, which is a highly toxic organic chlorine compound, or carbon monoxide (CO) and methanol as raw materials.
On the other hand, in recent years, a synthesis scheme has been developed for obtaining dimethyl carbonate by using CO2 in a supercritical state and acetone dimethyl acetal (also referred to as 2,2-dimethoxypropane) as raw materials. In such a synthesis scheme, CO2 in a supercritical state is allowed to react with acetone dimethyl acetal over a dibutyltin catalyst under conditions of 30 MPa and 180° C. However, since the dibutyltin catalyst is highly toxic and relatively high in cost, and also because the reaction conditions are tough, it cannot be said that this method is industrially favorable. Furthermore, the yield of dimethyl carbonate in this reaction is about 5%, so that this method is disadvantageous in terms of economy.
As another conventional art, Japanese Patent Provisional Publication No. 6-025104 (No. 025104/1994) has disclosed a method in which dimethyl carbonate is produced continuously by a gas-phase reaction in the presence of a solid catalyst consisting of a platinum-group metal or a compound thereof by using carbon monoxide and methyl nitrite as raw materials. However, this method has a problem of inferior handleability because it uses toxic CO as a raw material.
Japanese Patent Provisional Publication No. 7-069995 (No. 069995/1995) has disclosed a method in which dimethyl carbonate is produced by continuously subjecting carbon monoxide and methyl nitrite to a gas-phase reaction over a heterogeneous catalyst containing a platinum-group metal. However, because this method also uses toxic CO as a raw material, it has a problem of inferior handleability.
Also, Japanese Patent Provisional Publication No. 10-036297 (No. 036297/1998) has disclosed a method in which dimethyl carbonate is produced by transesterification using ethylene carbonate and ethanol. However, since the reaction selectivity is greatly affected by process conditions such as reaction temperature, this method has a problem in that a large-scale system for strictly controlling the reaction conditions is needed.